1. Field of the Invention
The present invention relates to a light emitting device comprising translucent ceramic plates.
2. Description of the Related Art
Solid state light emitting devices such as light emitting diode (LED), organic light emitting diode (OLED) or sometimes called organic electroluminescent device (OEL), and inorganic electroluminescent device (IEL) have been widely utilized for various application such as flat panel display, indicator for various instrument, signboard, and ornamental illumination, etc. As the emission efficiency of these light emitting devices continues to improve, applications that require much higher luminance intensity, such as automobile headlights and general lighting, may soon become feasible. For these applications, white-LED is one of the promising candidates and have attracted much attention.
Conventional white-LED have been manufactured based on the combination of blue-LED and yellow light-emitting YAG phosphor powder dispersed in plastic encapsulant resin like epoxy and silicone as disclosed in U.S. Pat. No. 5,998,925 and U.S. Pat. No. 6,069,440. Typical device structures are shown in FIGS. 1A and 1B. However, since the particle size of YAG phosphor powder utilized for this system is around 1-10 μm, the YAG powders dispersed in the encapsulant resin medium can cause strong light scattering. As a result, a considerable portion of both the incident light from the blue LED and the yellow emitting light from YAG powders ends up being back scattered and dissipated as a loss of white light emission as shown in FIG. 2.
One response to this problem is to form a monolithic ceramic member of wavelength converting material. However, these ceramic members can also suffer from decreased luminosity due to air voids formed during the formation of the ceramic. These air voids can result in reduced transparency to radiation emitted by the LED and/or increased levels of backscattering. Since the refractive index of the air as compared to the ceramic material is relatively large (on the order of 0.5 to 1.0), minute amounts of these air pockets can cause disproportionately large amount of backscattering.
One response to this problem of increased backscattering was to manipulate the refractive index of the materials used in the luminescent layer. For example, the refractive index of the medium in which the phosphor particles are embedded was increased to more closely match the refractive index of the phosphor particles (US 2003/0227249). De Graaf, et al (WO 2006/097876) describes a composite structure embedded in a matrix that is a ceramic composite structure comprising a polycrystalline ceramic alumina material. Sakata, et al (US 2006/0124951) describes a solidified body comprising two or more matrix phases with respective components being two or more oxides. However increased levels of non-emissive materials can still result in increased levels of scattering centers, and reducing the overall luminosity of the ceramic plate.
Another response to reduce the backscattering was to increase the transparency of the material by reducing the scattering centers, and thus minimize the resultant backscattering. However, a highly transparent ceramic plate without scattering centers still maintains a high differential in the refractive index as compared with the refractive index of the air or the encapsulant resin. This high differential results in an increased critical angle at the air/ceramic interface or at the encapsulant resin/ceramic interface, which results in a higher level of radiation being trapped within the ceramic plate due to total internal reflection. Such high levels of reflected radiation are often seen as an apparent illumination of the lateral edges of the ceramic. Thus while the transparency has been improved, the overall luminosity of the device could actually have been reduced.
Still another effort to reduce the back scattering involves preparing nano-sized YAG phosphor particles with particle sizes far smaller than the wavelength of visible light. For example, as disclosed in R. Kasuya et al., “Characteristic optical properties of transparent color conversion film prepared from YAG:Ce3+ nanoparticles,” Applied Physics Letters, 91, 111916 (2007), it may be possible to attain scattering free transparent nano-composite for such color-changing medium for white-LED if nano-sized YAG phosphor particles with size of less than around 30 nm are uniformly dispersed into encapsulant resin. However, it is well known that luminance intensity (or internal quantum efficiency) tends to decrease if particle size reaches several tens of nanometers or less. Such small particles have net increased surface area to bulk ratio, which means higher total population of unfavorable surface defect sites compared to the same amount of encapsulated micron-sized YAG phosphor particles. As a result, a white LED light output of a LED with nano-sized YAG phosphor particles actually exhibits lower efficiency than a micron-sized phosphor powder based device. Although the backscattering loss can be minimized in a nano-sized phosphor powder based device, the inferior luminance property can surmount the advantage in reduced backscattering. In addition, it is not easy to prepare transparent or translucent mono-dispersed nano-composite due to nano-sized particles' overwhelming strong tendency to aggregate. Thus far, there is no effective way to enhance the light output from a white-LED by minimizing the backscattering loss without sacrificing luminance efficiency of phosphor powders.